1. Field of the Invention
The present invention relates to technology for programming non-volatile memory.
2. Description of the Related Art
Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Both EEPROM and flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states (an erased state and a programmed state). When programming an EEPROM or flash memory device, typically a program voltage is applied to the control gate and the bit line is grounded. Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised so that the memory cell is in the programmed state. More information about programming can be found in U.S. patent application Ser. No. 10/379,608, titled “Self Boosting Technique,” filed on Mar. 5, 2003; and in U.S. patent application Ser. No. 10/629,068, titled “Detecting Over Programmed Memory,” filed on Jul. 29, 2003, both applications are incorporated herein by reference in their entirety.
A multi-state flash memory cell is implemented by identifying multiple, distinct allowed threshold voltage ranges separated by forbidden voltage ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits.
Typically, the program voltage applied to the control gate is applied as a series of pulses. The magnitude of the pulses is increased with each successive pulse by a predetermined step size (e.g. 0.2 v). In the periods between the pulses, verify operations are carried out. As the number of programmable states increase, the number of verify operations increases and more time is needed. One means for reducing the time burden of verifying is to use a more efficient verify process, such as the process that is disclosed in U.S. patent application Ser. No. 10/314,055, “Smart Verify for Multi-State Memories,” filed Dec. 5, 2002, incorporated herein by reference in its entirety. However, there is a consumer demand for memory devices that program as fast as possible. For example, a user of a digital camera that stores images on a flash memory card does not want to wait between pictures.
In addition to programming with reasonable speed, to achieve proper data storage for a multi-state memory cell, the multiple ranges of threshold voltage levels of the multi-state memory cell should be separated from each other by sufficient margin so that the level of the memory cell can be programmed and read in an unambiguous manner. Additionally, a tight threshold voltage distribution is recommended. To achieve a tight threshold voltage distribution, small program steps typically have been used, thereby, programming the threshold voltage of the cells more slowly. The tighter the desired threshold distribution, the smaller the steps and the slower the programming process.
One solution for achieving tight threshold distributions without unreasonably slowing down the programming process is to use a two phase programming process. The first phase, a coarse programming phase, includes attempts to raise the threshold voltage in a faster manner and paying relatively less attention to achieving a tight threshold distribution. The second phase, a fine programming phase, attempts to raise the threshold voltage in a slower manner in order to reach the target threshold voltage while also achieving a tighter threshold distribution. Example of coarse/fine programming methodologies can be found in the following patent documents that are incorporated herein by reference in their entirety: U.S. patent application Ser. No. 10/766,217, “Efficient Verification for Coarse/Fine Programming of Non-Volatile Memory” filed Jan. 27, 2004; U.S. patent application Ser. No. 10/051,372, “Non-Volatile Semiconductor Memory Device Adapted to Store A Multi-Valued Data in a Single Memory Cell,” filed Jan. 22, 2002; U.S. Pat. No. 6,301,161; U.S. Pat. No. 5,712,815; U.S. Pat. No. 5,220,531; and U.S. Pat. No. 5,761,222.
As memory devices become smaller and more dense, the need for tighter threshold distributions and reasonable program times has increased. Although the coarse/fine programming methodology provides a solution to some existing issues, there is further need to improve the coarse/fine programming methodology to provide the desired tighter threshold distributions and reasonable program times.